The metallic tube in a hot finishing state is delivered to a cold working process, when the metallic tube does not satisfy requirements in quality, strength, or dimensional accuracy. Generally, examples of the cold working process include a cold drawing process in which a die and a plug or a mandrel bar are used and a cold rolling process in which a cold pilger mill is used.
In the cold rolling process with the cold pilger mill, diameter reducing rolling is performed to a mother tube between a pair of rolls having a circumferentially-tapered groove die whose calibers are gradually reduced in a circumferential direction and a tapered mandrel bar whose diameters are gradually reduced toward its front end in a lengthwise direction. That is, the grooves are provided over the circumferences of the pair of rolls, and the grooves have such configuration that calibers of the grooves become narrowed as the rolls are rotated. The roll is repeatedly advanced and retreated along the tapered mandrel bar while rotated, whereby the rolling is performed to the mother tube between the rolls and the mandrel bar (for example, see “Iron and Steel Handbook third version” vol. 3, (2) Steel Bar, Steel Tube, and Rolling Common Facilities).
FIG. 1 is a view showing a rolling principle of the cold pilger mill, FIG. 1(a) is an explanatory view showing a start point of a forward stroke, and FIG. 1(b) is an explanatory view showing a start point of a backward stroke. As shown in FIG. 1, in the cold pilger mill, a pair of rolls 2 and a tapered mandrel bar 4 are used according to an outside diameter do and a wall thickness to of a mother tube 1 and an outside diameter d and a wall thickness t of a rolled tube 5 of a product. The roll 2 has a tapered groove die 3 whose calibers are gradually reduced from an engaging entry side of each of the pair of rolls toward a finishing exit side. The diameters of the tapered mandrel bar 4 are gradually reduced from the engaging entry side toward the finishing exit side. Forward and backward strokes in which the wall thickness is decreased while the diameter of the mother tube 1 is reduced are repeated.
At a start point of the forward stroke and a start point of the backward stroke in the reciprocating motion, a turn by about 60° and a feed ranging from about 5 to about 15 mm are intermittently imparted to the hollow-shell (mother tube 1), so that the rolling is performed on a new work area successively.
The cold rolling with the cold pilger mill is capable of applying an extremely high working rate to the hollow-shell, and tenfold elongation can be performed. Additionally, the cold rolling has a large effect on correcting an eccentricity of the wall thickness of tube, a further reducing process is not required, and the cold rolling features a high production yield. However, the cold rolling with the cold pilger mill has a disadvantage of extremely low productivity compared with the cold drawing process. Therefore, the cold rolling with the cold pilger mill is mainly suitable to cold working of high grade tubes, such as stainless tubes and high alloy steel tubes, in which raw materials and intermediate treatment costs are expensive. In a copper and copper alloy manufacturing industry, high-efficiency production is realized by three-strand rolling, and the cold pilger mill becomes a core production process for copper and copper alloy products.
In the cold drawing process, a tube end of the mother tube is swaged by a swaging machine, acid pickling is performed to remove a surface scale and the like, and lubricating treatment is performed to draw the mother tube through a die. Examples of the cold drawing process include plug drawing, drawing by using a floating plug, drawing by using a mandrel bar, and die drawing without a plug. All the cold drawing processes are performed by diameter reduction working with the die.
FIG. 3 is an explanatory view of the conventional drawing in which an outside diameter is reduced, FIG. 3(a) shows the plug drawing, and FIG. 3(b) shows drawing by using the mandrel bar.
The plug drawing shown in FIG. 3(a) is a most common drawing process. In the plug drawing, a plug 23 supported by a plug supporting rod 24 is inserted into the mother tube 1, the tube end of the mother tube 1 is gripped with a chuck 6, and the mother tube 1 is drawn through a die 22 in the direction shown by an arrow X of FIG. 3. The plug drawing has the advantages in plug exchange and operation efficiency, and the plug drawing also allows the substantial working rate.
The drawing by using the mandrel bar shown in FIG. 3(b) is a process in which a mandrel bar 25 is inserted into the mother tube 1 and the mother tube 1 is drawn through the die 22 like the plug drawing. In the drawing by using the mandrel bar, because the tube inner surface is processed by the mandrel bar 25, a product having a glossy inner surface can be produced with high dimensional accuracy even for the small diameter tube. Therefore, the drawing by using the mandrel bar is used in producing high grade tubes for use in a nuclear power plant and the like.
Most of drawing machines used in the cold drawing are driven by a motor with a chain, but some drawing machines are hydraulically-operated (either oil or water).
In the metallic-tube cold drawing process, there occurs frictional drag between the outer surface of the hollow-shell and the die surface and between the inner surface of the hollow-shell and the surface of the plug or mandrel bar, and the drawing is performed against the frictional drag. Therefore, tension is generated in a longitudinal direction of the hollow-shell. With the increase in tensile stress given by dividing the tension force by a post-drawing sectional area, a phenomenon that the tube outside diameter after drawing becomes smaller than the inside diameter of die is generated, and the in-processing tube breaks when the tensile stress reaches a deformation resistance of the hollow-shell. Obviously, as the wall thickness of the tube is thinned, the tensile stress is increased in a longitudinal direction and the tube becomes likely to break. Therefore, there is a limitation in a reduction rate of the wall thickness. Accordingly, in the drawing with the large reduction rate of the wall thickness, the number of drawing passes is increased and the repeated drawing operation is required, so that the lubricating work is required in each case to result in the cost increase. In the case that the large work hardening is generated in the hollow-shell, annealing process is also required.